After thinking the matter over a bit, here's what I think needs to be changed, at a minimum, to make "The Survivor" worth reading.

1) The house shouldn't be clean, merely less dusty than the narrator expects.2) Completely eliminate the name-dropping of gods and entities in the found papers - it has nothing to do with the story, and merely constitutes Derleth rather clumsily trying to mark the story as Lovecraftian. Change them to a brief mention of peculiar gods and unspeakable rites referencing forbidden topics found in the Necronomicon.3) The narrator never sees the lizard-doctor in person.

Instead, one night the narrator hears peculiar noises in the house, and the reek of serpentine musk becomes overpowering. With his heart in his throat, the narrator takes a flashlight and gun and stealthily begins to search the place. He turns a corner - and sees a dark and looming shape at the end of the corridor! He panics and shoots the figure repeatedly! With no visible effect. Then something hits him on the back of the head, and as he passes into darkness he senses something slither past him.

When he comes to, the laboratory has been emptied of notes and certain chemicals. The gun is also missing. And propped up at the end of the corridor, with several bullet holes in it, is a skin stuffed with newspaper. A snakeskin - in the form of a man!

The narrator gets the cold chills and flees the accursed abode. As he passes through the front doors with his possessions in two suitcases, he hears demoniac laughter echoing from the weed-encrusted well... laughter mixed with a terrible hissing.

Partly the story relies on the reader fearing snakes too much--it is meant to show a debasement of the Doctor's humanity--but if you don't hate snakes it loses some of its oomph. So, you need a more credible reason for anxiety.